This invention relates to a process for making a silicon aluminum oxynitride refractory material, and more particularly a process wherein at least a portion of initial reactants are converted to at least a portion of effective reactants in a first heating step in the presence of nitrogen and effective reactants are converted to a silicon aluminum oxynitride refractory material in a second heating step.
Silicon aluminum oxynitride refractory materials, and more particularly materials in the Si.sub.3 N.sub.4 -AlN-Al.sub.2 O.sub.3 -SiO.sub.2 system, are of ever-increasing interest for refractory applications. For ease of identification, compositions within this system are referred to as SiAlON, and a number of different phases of SiAlON have been produced and identified. For example, Jack et al U.S. Pat. No. 3,991,166 describes one phase and methods of making it, the phase having the general formula Si.sub.6-z Al.sub.z O.sub.z N.sub.8-z where z is greater than zero and less than or equal to five. Various compositions within the bounds of the general formula taught by Jack et al may be produced, and each has a crystalline structure similar to beta-Si.sub.3 N.sub.4 and is consequently identified as beta'-SiAlON. Beta'-SiAlON can be defined as a solid solution of Al.sub.2 O.sub.3 within a matrix of Si.sub.3 N.sub.4. The compositional limits of reactants, referred to as effective reactants, to produce beta'-SiAlON may be seen by referring to FIG. 2. The compositional amounts of Si.sub.3 N.sub.4, AlN and Al.sub.2 O.sub.3 for any beta'-SiAlON formulation may be determined by referring to line AB which is a plot of the compositions of the aforesaid compounds to produce a beta'-SiAlON having the general formula Si.sub.6-z Al.sub.z O.sub.z N.sub.8-z where z is greater than zero and less than or equal to five.
Another phase, known as y-phase SiAlON represented by the formula SiAl.sub.4 O.sub.2 N.sub.4, is described in an article entitled "Review: SiAlONs and Related Nitrogen Ceramics", published in Journal of Material Sciences, 11, (1976) at pages 1135-1158. Compositions of SiAlON within a given phase and from phase to phase demonstrate varying characteristics, for example, variances in density, which effect their preferential use in a given application.
Thus far, of all the SiAlON materials, the beta'-SiAlONs have generated the greatest interest because their refractory properties and corrosion resistance characteristics are comparable to other nitride refractories such as silicon nitride and silicon oxynitride. Beta'-SiAlON compositions offer a distinct advantage over silicon nitride and silicon oxynitride for making a refractory, however, because some of the compositions of beta'-SiAlON material can be used to produce a high density refractory by conventional sintering techniques. To produce high density refractories from silicon nitride or silicon oxynitride requires the use of pressure sintering techniques.
A number of processes for making silicon aluminum oxynitride refractories and refractory materials have been suggested. Weaver U.S. Pat. No. 3,837,871 describes a method for producing a product having a substantial amount of what the patentee believes to be the quaternary compound silicon aluminum oxynitride which has a structure similar to that of beta Si.sub.3 N.sub.4 but with an expanded lattice structure. Weaver's method of making the described product is hot pressing Si.sub.2 ON.sub.2 (silicon oxynitride) in the presence of varying amounts of aluminum.
Kamigaito et al U.S. Pat. No. 3,903,230 describes a method of making a silicon aluminum oxynitride ceramic by sintering or hot pressing a mixture of finely divided powders of silicon nitride, alumina and aluminum nitride.
Cutler U.S. Pat. No. 3,960,581 describes a process for producing SiAlON by reacting silicon and aluminum compounds in the presence of carbon and nitrogen. Cutler teaches and stresses the importance of using a reactant material having the silicon and aluminum compounds intimately combined prior to nitriding in order that aluminum oxide is intimately dispersed throughout silicon nitride in the final product. Suggested reactant materials are clay, rice hulls having a solution containing a dissolved aluminum salt absorbed therein, and a precipitate of aluminum and silicon salts. In each case Cutler emphasizes that the silicon and aluminum compound reactants are intimately combined prior to nitriding to produce SiAlON. Further, in the process as taught by Cutler excess carbon and unreacted silicon dioxide must be removed from the mixture after the mixture is nitrided.
Jack et al U.S. Pat. No. 3,991,166 describes a beta'-SiAlON product produced by sintering a mixture of alumina or a compound which decomposes to produce alumina and silicon nitride. Another method of producing beta'-SiAlON as described by Jack et al is nitriding silicon powder in the presence of alumina powder.
It may be noted that several of the foregoing processes employ silicon nitride or silicon oxynitride as reactants. Neither of these compounds is found in nature and they are relatively expensive to produce. Cutler's process provides for the use of reactants found in nature but does not employ a two-step heating process in producing beta'-SiAlON.
It would be advantageous, therefore, to provide a process whereby readily available and relatively inexpensive initial reactant materials comprising Al.sub.2 O.sub.3 and SiO.sub.2 are nitrided to make silicon aluminum oxynitride materials without the necessity of further processing in removing excess carbon and/or silica.